Field of the Invention
The present invention relates to an antenna device and, more particularly, to an antenna device suitable for NFC (Near Field Communication). The present invention further relates to a coil component used in such an antenna device.
Description of Related Art
In recent years, a mobile wireless device such as a smartphone is equipped with an RFID (Radio Frequency Identification: individual identification by radio waves) system, and further equipped with, as a communication means of the RFID, an antenna for performing the near field communication with a reader/writer and the like.
Further, such a mobile wireless device is provided with a metal shield in order to protect a built-in circuit from external noise and prevent unnecessary radiation of noise generated inside the device. In particular, recently, in view of thinning, light-weighting, durability against impacts such as drops, designability, and the like, a housing itself of the mobile wireless device is often made of a metal instead of a resin and serves also as the metal shield. However, the metal shield blocks radio waves in general, so that when an antenna needs to be provided, it needs to be disposed at a position not covered with the metal shield. Thus, when the metal shield covers a wide range, it is difficult to dispose the antenna.
To solve the above problem, an antenna device described in, e.g., Japanese Patent No. 4,941,600 is an NFC antenna suitable for an RFID system and includes a loop or spiral coil conductor, a conductor layer having an aperture and a slit continuous with the aperture, and a magnetic sheet disposed at a position further than the coil conductor from the conductor layer, wherein an opening of the coil conductor overlaps with the aperture of the conductor layer in a plan view. In this antenna device, current flows in the conductor layer so as to block a magnetic field generated by the flowing of the current in the coil conductor. Then, the current flowing around the aperture of the conductor layer passes around a slit, with the result that the current also flows around the conductor layer by an edge effect. Thus, a magnetic field is generated also from the conductor layer, and the conductor layer enlarges a magnetic flux loop, whereby a communication distance between the antenna device and a counterpart antenna can be extended.
Although the above conventional antenna device described in Japanese Patent No. 4,941,600 is so-called a planar coil antenna, there is known a three-dimensional solenoid antenna. An antenna of this type is easily increased in inductance than the planar coil antenna, can generate more magnetic lines, and is easily reduced in size. For example, an antenna device described in Japanese patent application Laid-Open No. 2014-36437 has a configuration in which a magnetic core is sandwiched between patterns formed on respective flexible substrates, and the patterns are connected by soldering.
In a case where the housing itself of the mobile wireless device serves also as the metal shield as described above, a consideration needs to be made to prevent an antenna from being affected by the metal shield even if the antenna is a solenoid antenna, and when the metal shield covers a wide range, it is difficult to dispose the antenna. Further, an aperture size of a known solenoid antenna wound around an outer peripheral surface of a winding core of a magnetic core or the like is restricted by a cross-sectional size of the magnetic core, making it difficult to increase the aperture size. Further, the solenoid antenna has too much directivity and, in addition, when a size of the antenna is reduced, it cannot generate a large magnetic flux loop that can cross a counterpart antenna, resulting in a short communication distance, which poses a practical problem for the solenoid antenna as a near field communication antenna.